Downhole milling tools may be used to, for example, form casing windows or remove entire sections of downhole casing. Downhole milling tools may also be used to remove metallic debris—known as “junk”—that has fallen into the wellbore.
Downhole milling tools may include a tubular body having a plurality of equi-azimuthally spaced cutting blades coupled to the body. Each cutting blade has a forward surface facing the direction of rotation of the tool which is dressed with a cutting material (e.g., one or more cutting elements disposed in an outer surface of the cutting blade). The cutting material may include or define a protruding ridge or chip breaker, which is a projection that limits the length of swarf or chip cut by the leading cutting edge of the element.
Chip breakers are used to prevent or reduce “birdnesting,” which is the term given to the long spirals of swarf that are cut from a tubular member (e.g., casing), that form into a conglomerate mass, which may restrict the flow of drilling mud about a tool, reduce the rate of penetration of the tool, and reduce the ability to carry cuttings back to the surface. Chip breakers may control the size of chips formed by the cutting element to increase the speed and efficiency of milling.